Field of the Invention
The present invention relates to channel state feedback (CSF) estimation techniques in wireless communication systems.
Background Art
Wireless communication system transmissions can be classified as either downlink (DL) or uplink (UL) transmissions. Downlink transmissions refer to signals transmitted from a base station (e.g., an eNodeB) to a mobile device (e.g., a user equipment (UE)). Uplink transmissions refer to signals transmitted from the mobile device (e.g., the UE) to the base station (e.g., the eNodeB). To ensure reliable and consistent communication, wireless systems often track the channel state conditions and adjust communication parameters (e.g., power or modulation parameters) accordingly. As an example, in TD-LTE systems, UE's calculate channel state feedback (CSF) reports based on downlink signals.
In a time-division (TD) based wireless communication system, such as TD-LTE, a frame structure for transmissions can include a plurality of subframes. The subframes can be designated as either uplink or downlink subframes. The frame structure can also include at least one special subframe. FIG. 1 illustrates an exemplary special subframe 100 for a TD wireless communication system, such as TD-LTE. The special subframe 100 can include a downlink pilot time slot or transmission 110, a guard period 120, and an uplink pilot time slot or transmission 130. The uplink and downlink pilot transmissions 110 and 130 can include reference signals. The reference signals can be used for a number of purposes including channel measurements or maintaining synchronization. The guard period 120 can separate the uplink and downlink pilot transmissions 110 and 130 to account for the round trip delay experienced by transmissions between the mobile device and the base station, and to account for multipath delay. Hence, the duration of the guard period 120 can directly correlate to the cell size (i.e., the larger the cell size, the longer the guard period 120).
For many TD wireless communication systems, such as TD-LTE, the number of uplink and downlink subframes can be varied. That is, while the total number of subframes in a frame structure can be fixed, the number of individual uplink subframes and downlink frames within a particular frame can be adjusted, and can vary from frame to frame. Consequently, downlink subframes are not always transmitted consecutively. Specifically, downlink subframes can be discontinuous because downlink subframes can be separated by one or more uplink subframes and/or one or more special subframes. Transmission parameters for a downlink subframe can be set based on CSF information where the CSF information, generally, is based on prior downlink transmissions. However, CSF information for discontinuous downlink subframes may not be accurate because channel states can change during the gaps between downlink subframes, which can lead to deterioration in communication quality.
Existing CSF estimation techniques for handling discontinuous downlink subframes are not optimal for varying channel state conditions. One technique is to simply ignore the first downlink subframe received after a gap of uplink subframes and/or special frames. In doing so, information for determining a reliable CSF estimation can be wasted. Further, if the mobile device is operating in a power save mode, ignoring the first discontinuous downlink subframe may require the mobile to consume more power waiting and then subsequently processing a next received downlink subframe to generate a CSF estimation.
Accordingly, what is needed is an adaptive CSF estimation technique; in particular, an adaptive CSF estimation technique that can accommodate varying channel state conditions and discontinuous downlink subframes.